In long distance fiber optic communication systems it may be important to monitor the health of the system. For example, a monitoring may be implemented to detect faults or breaks in the fiber optic cable, faulty repeaters or amplifiers or other problems with the system.
Known monitoring techniques include use of optical time domain reflectometry (“OTDR”) equipment that may generate a test signal representing a predefined bit sequence. The OTDR equipment may transmit the test signal with the information signals, e.g. in wavelength division multiplexed system. The test signal may be returned to the OTDR equipment through reflection in a transmit path and/or through a return path within an amplifier or repeater. The OTDR equipment may then separate the returned test signal from the data signals, and process the test signal to examine the health of the transmission system.